Absolute position transmitters detect the position of a movable element, for example of a movable slide, and report this position on to a superordinate controller. Every position within the movement path is in this case detected precisely, for example using 12 bits. Thus, there is one, and only one, bit pattern which corresponds to each position of the movable element, and, conversely a quite specific section of the movement path corresponds to each bit pattern--within the measurement accuracy. The absolute position transmitter supplies the value "0" at one position on the movement path. This point need not in this case be arranged at one of the ends of the movement path, but may be located at any desired point. Passing through the position at which the absolute position transmitter emits the value "0" is called a zero crossing.
If it is intended for the movement element to drive to any desired new position within the movement path, the current ACTUAL position, which is known on the basis of the bit pattern supplied from the absolute position transmitter, is initially compared with the new REQUIRED position. The comparison of the two positions with one another is used to determine the direction in which the movement element must be moved. Furthermore, the new REQUIRED position is converted into its corresponding REQUIRED bit pattern and is loaded into a comparator. The absolute position transmitter continuously detects the ACTUAL position of the movement element and reports the bit pattern corresponding to it to the comparator at constant time intervals. The comparator emits a switching signal to the control element, which moves the movement element, when the ACTUAL bit pattern for the first time becomes less than or greater than the REQUIRED bit pattern, depending on the movement direction. The switching condition is thus that the ACTUAL bit pattern becomes greater than or equal to, or less than or equal to, the REQUIRED bit pattern.
In practice, it is normally not possible to stop the movement element abruptly on reaching the new REQUIRED position. The comparator is thus normally not loaded initially with the bit pattern which corresponds to the new REQUIRED position but with the bit pattern which corresponds to a position which is a specific distance in front of the REQUIRED position to be driven to, in the movement direction. On reaching this position, the movement element is initially changed over from normal drive to inching, and the bit pattern which corresponds to the genuine REQUIRED position is then loaded into the comparator, and the drive, which displaces the movement element, is switched off completely on reaching the REQUIRED position. However, these differences are irrelevant for the problem on which the present case is based. It is thus always assumed in the following text that the REQUIRED position is the position at which the comparator is intended to respond. The response of the comparator can then, depending on the requirement, initiate the changeover of the control drive to inching, or the stopping of the control drive.
However, a number of problems result from the fact that the zero crossing of the absolute position transmitter may lie between the new REQUIRED position and the ACTUAL position of the movement element when the new REQUIRED bit pattern is set. On the one hand, the comparator cannot be loaded immediately with the new REQUIRED bit pattern, since the zero crossing must be detected first. Otherwise, the comparator would, in fact, respond immediately although the position to be driven to had not yet been reached. Furthermore, the comparator also responds when an accidental zero crossing of the absolute position transmitter occurs after the loading of the comparator. However, the major problem is presented by the situation in which the REQUIRED position is located shortly before the transmitter zero crossing. Specifically, as a result of the sampling, it is possible that the last sample before the zero crossing will no longer detect the switching point. In this case, the system moves past the REQUIRED position without the comparator responding.
In the prior art, the bit pattern supplied from the absolute position transmitter is thus always converted into the position corresponding to it and is compared with the new REQUIRED position. The switching signal is now emitted on reaching or moving past the new REQUIRED position. This procedure admittedly avoids the problems mentioned above in comparing the bit patterns with one another, but is time-consuming and cumbersome. Furthermore, it is not possible to ensure a constant reaction time reliably.